JOHANNESBURG – A newly discovered dinosaur species that roamed the Earth about 200 million years ago may help explain how the creatures evolved into the largest animals on land, scientists in South Africa said Wednesday. The Aardonyx celestae was a 23-foot- 7-meter- long small-headed herbivore with a huge barrel of a chest. It walked on its hind legs but also could drop to all fours, and scientists told reporters that could prove to be a missing evolutionary link. This is a species "that no one has seen before and one that has a very significant position in the family tree of dinosaurs," said Australian paleontologist Adam Yates. Yates, who is based at the University of the Witwatersrand's Bernard Price Institute for Paleontological Research, led the research with a number of other local and international scientists. Their findings were published Wednesday in the Proceedings of The Royal Society B, a London-based peer-reviewed journal. The Aardonyx celestae species dates back to the early Jurassic period. Yates said the creature found in South Africa stood nearly 6 feet about 1.7 meters high at the hip and weighed about 1,100 pounds 500 kilograms. It was about 10 years old when it died, and its death may have been caused by drought. The newly discovered species shares many characteristics with the plant-eating herbivores that walked on two legs, Yates said. But the new species also has similar attributes to dinosaurs known as sauropods, or brontosaurs, that grew to massive sizes and went about on all fours with long necks and whip-like tails. "The discovery of Aardonyx helps to fill a marked gap in our knowledge of sauropod evolution, showing how a primarily two-legged animal could start to acquire the specific features necessary for a life spent on all-fours," said Paul Barrett, a paleontologist at the British Natural History Museum who assisted on the dig that led to the finding but was not directly involved in the research. Why and how dinosaurs grew into such massive creatures is a question that scientists have been trying to answer for a long time. Walking on all fours allowed animals to carry more weight, and size was often their only defense against sharp-toothed carnivores, said one of the report's co-authors, Matthew Bonnan of Western Illinois University, by video conference from the United States. The discovery of the new species was made by postgraduate student Marc Blackbeard, who was excavating two sites in central South Africa about five years ago. It was a site that had been largely ignored by scientists who felt the bone fragments found there would prove to be a common dinosaur species found across the country. On the first day of excavation, a bone too large to belong to this ordinary species was found. "We knew we had something new, something very, very exciting," Yates said. The scattered bones were collected and cleaned of the heavy cement-like rock that clung to them. Scientists then began the slow process of studying the bones and trying to order them. They were pleased with how much of the skeleton they could reconstruct and especially that a large part of the skull was found. Bonnan said he also was thrilled to be part of this discovery: "It has been a childhood dream to discover a new dinosaur," he said. ___